Historically, most commercially-available, heat-shrinkable, food-packaging films have been polyethylene-based. Polyethylene, particularly ethylene/alpha-olefin copolymers, are relatively inexpensive, have relatively low melting points, are readily heat-sealable, and are readily oriented in the solid state, i.e., at a temperature between the glass transition temperature of the polymer and the melting point of the polymer. Such films can be provided with a relatively high total free shrink value at, for example, 85° C. (185° F.).
In the last few years, semi-crystalline polyamide-based shrink films have begun to compete against polyethylene-based shrink films for the packaging of fresh meat products, even though polyamide is more expensive than polyolefin. One reason is that semi-crystalline polyamide-based shrink films can provide higher impact strength per mil than polyethylene-based films. Unlike patchless bags made from polyethylene-based films, patchless bags made from semi-crystalline polyamide-based films can provide adequate protection for some abusive food products, such as some bone-in meat products.
However, semi-crystalline polyamide-based shrink films are difficult to produce because it is difficult to carry out the solid-state orientation of the semi-crystalline polyamide necessary to impart the desired degree of low-temperature heat-shrinkability. Recently a turn-key production line has been developed and offered for sale by Kuhne Anlagenbau GMBH. Kuhne Anlagenbau GMBH developed this production line concurrently with its development of, and for the manufacture of, a turn-key semi-crystalline polyamide-based multilayer heat-shrinkable packaging film. This Kuhne turn-key production line includes an elaborate quenching means beneath the extrusion die. The quenching means employs vacuum around the exterior of the annular extrudate as it emerges from the die, while at the same time applying water to the extrudate shortly after it emerges from the die. The vacuum is employed so that the annular extrudate does not collapse upon itself before it solidifies enough that it is fully quenched. The vacuum around the outside of the extrudate affects the quenching of the extrudate, because the vacuum draws the applied quenching water from the surface of the extrudate.